Method for performing handover, user equipment, and radio communication system

ABSTRACT

A mobile station in a wireless communication network. The mobile station includes a radio communication that transmits an access request message to a base station via a first communication resource, and receives a timing adjustment in response to the access request message from the base station. The mobile station also includes an adjustment value storage unit that stores the timing adjustment, and a control unit that adjusts access timing corresponding to a second communication resource based on the timing adjustment value stored in the adjustment value storage unit. The radio communication unit then communicates with the base station via the first communication resource and the second communication resource.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims the benefit ofpriority under 35 U.S.C. §120 from U.S. application Ser. No. 15/136,507,filed Apr. 22, 2016, which is a continuation of U.S. application Ser.No. 13/514,163, now U.S. Pat. No. 9.374,838, filed Jun. 6, 2012, theentire contents of each of which is incorporated herein by reference.U.S. application Ser. No. 13/514,163 is a national stage ofInternational Application No. PCT/JP2010/007037, filed Dec. 2, 2010,which is based on and claims the benefit of priority under 35 U.S.C.§119 from prior Japanese Patent Application No. 2009-285374, filed Dec.16, 2009.

TECHNICAL FIELD

The present invention relates to a method for performing a handover, auser equipment, and a radio communication system.

BACKGROUND ART

In Long Term Evolution-Advanced (LTE-A), which is the next-generationcellular communication standard that is discussed in Third GenerationPartnership Project (3GPP), introduction of technology called carrieraggregation (CA) has been studied. The carrier aggregation is technologythat forms a communication channel between a user equipment (UE) and abase station (BS, or evolved Node B (eNB)) by aggregating a plurality offrequency bands that are supported in LTE, for example, and therebyimproves communication throughput. Each frequency band included in onecommunication channel by the carrier aggregation is called a componentcarrier (CC). The bandwidths of frequency bands that are available inLTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz.Accordingly, if five bands of 20 MHz are aggregated as componentcarriers, a communication channel of 100 MHz in total can be formed.

Component carriers that are included in one communication channel in thecarrier aggregation are not necessarily contiguous to one another in thefrequency direction. The mode in which component carriers are arrangedcontiguous to one another in the frequency direction is called acontiguous mode. On the other hand, the mode in which component carriersare arranged not contiguous to one another is called a non-contiguousmode.

Further, in the carrier aggregation, the number of component carriers inan uplink and the number of component carriers in a downlink are notnecessarily equal. The mode in which the number of component carriers inan uplink and the number of component carriers in a downlink are equalis called a symmetric mode. On the other hand, the mode in which thenumber of component carriers in an uplink and the number of componentcarriers in a downlink are not equal is called an asymmetric mode. Forexample, in the case of using two component carriers in an uplink andthree component carriers in a downlink, it is asymmetric carrieraggregation.

Further, in LTE, any one of frequency division duplex (FDD) and timedivision duplex (TDD) can be used as duplex operation. Because thedirection of a link (uplink or downlink) of each component carrier doesnot change in time in FDD, FDD is better suited to the carrieraggregation compared to TDD.

A handover, which is a basic technique for achieving the mobility of auser equipment in the cellular communication standard, is one ofimportant subjects in LTE-A. In LTE, a user equipment measures acommunication quality over a channel with a serving base station (acurrently connected base station) and communication qualities withperipheral base stations and transmits a measurement report containingmeasurements to the serving base station. Receiving the measurementreport, the serving base station determines whether to execute ahandover based on the measurements contained in the report. Then, if itis determined that a handover is to be executed, a handover is carriedout among a source base station (the serving base station before ahandover), the user equipment, and a target base station (a serving basestation after a handover) in accordance with a prescribed procedure(e.g. cf. Patent Literature 1 below)

CITATION LIST Patent Literature

[PTL 1]

Japanese Unexamined Patent Application Publication No. 2009-232293

SUMMARY OF INVENTION Technical Problem

However, no case has been reported where active consideration is givento how to carry out a handover procedure in a radio communicationinvolving the carrier aggregation.

For example, an initial access from a user equipment to a target basestation during a handover procedure is performed as a random access overa random access channel which is preset at a prescribed position ofcommunication resources. However, a random access inherently has apossibility of a delay due to signal collision, retry or the like.Further, adjustment of transmission timing based on timing of a randomaccess is the overhead of a communication. Thus, in a radiocommunication involving the carrier aggregation, simply repeating randomaccesses the same number of times as the number of component carriersraises the possibility of degradation of service quality such as anaccumulation of delays due to a failure of a random access or anincrease in overhead.

In light of the foregoing, it is desirable to provide a novel andimproved method for performing a handover, user equipment, and radiocommunication system that can minimize the degradation of servicequality due to a random access during a handover procedure in a radiocommunication involving the carrier aggregation.

Solution to Problem

One exemplary embodiment includes a mobile station in a wirelesscommunication network. The mobile station including a radiocommunication unit configured to transmit an access request message to abase station via a first communication resource, and receive a timingadjustment in response to the access request message from the basestation; an adjustment value storage unit configured to store the timingadjustment; and a control unit configured to adjust access timingcorresponding to a second communication resource based on the timingadjustment value stored in the adjustment value storage unit, whereinthe radio communication unit is configured to communicate with the basestation via the first communication resource and the secondcommunication resource.

The first communication resource may be a first component carrier andthe second communication resource may be a second component carrier. Thesecond component carrier may be in close proximity to the firstcomponent carrier. The second component carrier may be adjacent to thefirst component carrier. The second component carrier may be within apredetermined threshold of the first component carrier.

The control unit may be configured to acquire synchronization with thebase station via the first communication resource, and the radiocommunication unit may be configured to transmit the access requestmessage based on the synchronization.

The timing adjustment may be based on a signal propagationcharacteristic between the mobile station and the base station, and thetiming adjustment may depend on a frequency of the first communicationresource.

Another exemplary embodiment includes a base station in a wirelesscommunication network. The base station including a radio communicationunit configured to receive an access request message from a mobilestation via a first communication resource, and transmit a timingadjustment in response to the access request message to the mobilestation; a control unit configured to assign a second communicationresource for communications with the mobile station, the secondcommunication resource being assigned based on a predeterminedrelationship with the first communication resource, wherein the radiocommunication unit is configured to communicate with the mobile stationvia the first communication resource and the second communicationresource.

The first communication resource may be a first component carrier andthe second communication resource may be a second component carrier. Thecontrol unit may be configured to assign a component carrier in closeproximity to the first component carrier as the second componentcarrier, assign a component carrier adjacent to the first componentcarrier as the second component carrier, and/or assign a componentcarrier which is within a predetermined threshold of the first componentcarrier as the second component carrier. The control unit may also beconfigured to assign a component carrier, which is one of a plurality ofavailable component carriers closest in proximity to the first componentcarrier, as the second component carrier.

Another exemplary embodiment is directed to a wireless communicationnetwork that includes a mobile station configured to transmit an accessrequest message to a base station via a first communication resource;the base station configured to receive the access request message andtransmit a timing adjustment in response to the access request messageto the mobile station; an adjustment value storage unit, at the mobilestation, configured to store the timing adjustment; a first controlunit, at the base station, configured to assign a second communicationresource for communications with the mobile station, the secondcommunication resource being assigned based on a predeterminedrelationship with the first communication resource; a second controlunit, at the mobile station, configured to adjust access timingcorresponding to the second communication resource based on the timingadjustment value stored in the adjustment value storage unit, whereinthe mobile station and the base station are configured to communicatevia the first communication resource and the second communicationresource.

Another exemplary embodiment is directed to a computer-readable mediumincluding computer program instruction, which when executed by a mobilestation in a wireless communication network, cause the mobile station toperform a method comprising: transmitting an access request message to abase station via a first communication resource; receiving a timingadjustment in response to the access request message from the targetbase station; storing the timing adjustment; adjusting an access timingcorresponding to a second communication resource based on the storedtiming adjustment value; and communicating with the base station via thefirst communication resource and the second communication resource.

Another exemplary embodiment is directed to a computer-readable mediumincluding computer program instruction, which when executed by a basestation in a wireless communication network, cause the base station toperform a method comprising: receiving an access request message from amobile station via a first communication resource; transmitting a timingadjustment in response to the access request message to the mobilestation; assigning a second communication resource for communicationswith the mobile station, the second communication resource beingassigned based on a predetermined relationship with the firstcommunication resource; and communicating with the mobile station viathe first communication resource and the second communication resource.

Another exemplary embodiment is directed to a handoff method performedby a mobile communication network, the handoff method comprising:transmitting, from a mobile station to a base station, an access requestmessage via a first communication resource; receiving, at the basestation, the access request message; transmitting, from the base stationto the mobile station, a timing adjustment in response to the accessrequest message; storing, at an adjustment value storage unit at themobile station, the timing adjustment; assigning, by the base station, asecond communication resource for communications with the mobilestation, the second communication resource being assigned based on apredetermined relationship with the first communication resource;adjusting, by the mobile station, access timing corresponding to thesecond communication resource based on the timing adjustment valuestored in the adjustment value storage unit; and performingcommunication between the base station and the mobile station via thefirst communication resource and the second communication resource.

Advantageous Effects of Invention

As described above, the method for performing a handover, the userequipment, and the radio communication system according to theembodiments of the present invention can minimize the degradation ofservice quality due to a random access during a handover procedure in aradio communication involving the carrier aggregation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sequence chart to describe a flow of a typical handoverprocedure.

FIG. 2 is an explanatory view to describe an example of a structure of acommunication resource.

FIG. 3A is a sequence chart to describe a procedure of acontention-based random access.

FIG. 3B is a sequence chart to describe a procedure of a contention-freerandom access.

FIG. 4 is a schematic view showing an outline of a radio communicationsystem according to an embodiment.

FIG. 5 is a block diagram showing an example of a configuration of auser equipment according to a first embodiment.

FIG. 6 is a block diagram showing an example of a detailed configurationof a radio communication unit according to the first embodiment.

FIG. 7 is a block diagram showing an example of a configuration of abase station according to the first embodiment.

FIG. 8 is an explanatory view to describe an allocation of componentcarriers by a target base station according to the first embodiment.

FIG. 9A is the first half of a sequence chart showing an example of aflow of a handover procedure according to the first embodiment.

FIG. 9B is the second half of a sequence chart showing an example of aflow of a handover procedure according to the first embodiment.

FIG. 10 is a block diagram showing an example of a configuration of auser equipment according to a second embodiment.

FIG. 11 is a block diagram showing an example of a configuration of abase station according to the second embodiment.

FIG. 12A is the first half of a sequence chart showing an example of aflow of a handover procedure according to the second embodiment.

FIG. 12B is the second half of a sequence chart showing an example of aflow of a handover procedure according to the second embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

Preferred embodiments of the present invention will be describedhereinafter in the following order.

-   1. Description of Related Art-   1-1. Handover Procedure-   1-2. Structure of Communication Resource-   1-3. Description of Issue Related to Random Access-   2. Outline of Radio Communication System-   3. Description of First Embodiment-   3-1. Exemplary Configuration of User Equipment-   3-2. Exemplary Configuration of Base Station-   3-3. Flow of Process-   3-4. Summary of First Embodiment-   4. Description of Second Embodiment-   4-1. Exemplary Configuration of User Equipment-   4-2. Exemplary Configuration of Base Station-   4-3. Flow of Process-   4-4. Summary of Second Embodiment

<1. Description of Related Art>

(1-1. Handover Procedure)

A technique related to the present invention is described hereinafterwith reference to FIGS. 1 to 3B. FIG. 1 shows a flow of a handoverprocedure in conformity with LTE in a radio communication not involvingthe carrier aggregation as an example of a typical handover procedure.In this example, a user equipment (UE), a source base station (sourceeNB), a target base station (target eNB), and a mobility managemententity (MIME) are involved in the handover procedure.

As a preliminary step toward a handover, the user equipment firstreports the channel quality of a communication channel between the userequipment and the source base station to the source base station (stepS2). The channel quality may be reported on a regular basis or when thechannel quality falls below a predetermined reference value. The userequipment can measure the channel quality of the communication channelwith the source base station by receiving a reference signal containedin a downlink channel from the source base station.

Then, the source base station determines the needs of measurement basedon the quality report received from the user equipment and, ifmeasurement is necessary, allocates measurement gaps to the userequipment (step S4).

Then, the user equipment searches for a downlink channel from aperipheral base station (i.e. performs cell search) during the periodsof the allocated measurement gaps (step S12) Note that the userequipment can recognize a peripheral base station to search according toa list that is provided in advance from the source base station.

When the user equipment acquires synchronization with a downlinkchannel, the user equipment performs measurement by using a referencesignal contained in the downlink channel (step S14). During this period,the source base station restricts an allocation of data communicationrelated to the user equipment so as to avoid occurrence of datatransmission by the user equipment.

Upon completion of the measurement, the user equipment transmits ameasurement report containing measurements to the source base station(step S22). The measurements contained in the measurement report may bethe average value or the central value of measured values over aplurality of times of measurement or the like. Further, the measurementsmay contain data about a plurality of frequency bands.

Receiving the measurement report, the source base station determineswhether or not to execute a handover based on the contents of themeasurement report. For example, when the channel quality of anotherbase station in the periphery is higher than the channel quality of thesource base station by a predetermined threshold or greater, it can bedetermined that a handover is necessary. In this case, the source basestation determines to carry out a handover procedure with the relevantanother base station as a target base station, and transmits a handoverrequest message to the target base station (step S24).

Receiving the handover request message, the target base stationdetermines whether it is possible to accept the user equipment accordingto the availability of a communication service offered by itself or thelike. When it is possible to accept the user equipment, the target basestation transmits a handover request confirm message to the source basestation (step S26).

Receiving the handover request confirm message, the source base stationtransmits a handover command to the user equipment (step S28). Then, theuser equipment acquires synchronization with the downlink channel of thetarget base station (step S32). After that, the user equipment makes arandom access to the target base station by using a random accesschannel in a given time slot (step S34). During this period, the sourcebase station forwards data addressed to the user equipment to the targetbase station (step S36). Then, after success in the random access, theuser equipment transmits a handover complete message to the target basestation (step S42).

Receiving the handover complete message, the target base stationrequests the MME to perform route update for the user equipment (stepS44). Upon updating the route of user data by the MME, the userequipment becomes able to communicate with another device through a newbase station (i.e. the target base station). Then, the target basestation transmits acknowledgement to the user equipment (step S46). Aseries of handover procedure thereby ends.

(1-2. Structure of Communication Resource)

FIG. 2 shows a structure of a communication resource in LTE as anexample of a structure of a communication resource to which the presentinvention is applicable. Referring to FIG. 2, the communication resourcein LTE is segmented in the time direction into radio frames each havinga length of 10 msec. One radio frame includes ten sub-frames, and onesub-frame is made up of two 0.5 msec slots. In LTE, the sub-frame is oneunit of an allocation of a communication resource to each user equipmentin the time direction. Such one unit is called a resource block. Oneresource block includes twelve sub-carriers in the frequency direction.Specifically, one resource block has a size of 1 msec with 12sub-carriers in the time-frequency domain. Throughput of datacommunication increases as a larger number of resource blocks areallocated for data communication on condition of the same bandwidth andtime length. Further, in such a structure of a communication resource, apart of radio frame with a given frequency band is reserved as a randomaccess channel. The random access channel can be used for an access to abase station by a user equipment that has changed from an idle mode toan active mode, for example, in addition to for an initial access to atarget base station in a handover procedure as described above.

(1-3. Description of Issue Related to Random Access)

FIGS. 3A and 3B are sequence charts to further describe general randomaccess procedures. FIG. 3A shows a procedure of a contention-basedrandom access.

Referring to FIG. 3A, the user equipment first transmits a random accesspreamble to the target base station (step S52). The random accesspreamble is selected from 64 different sequences. Because the 64sequences are orthogonal to one another, even when the target basestation receives different sequences at the same timing with the samefrequency, it can separate those sequences. However, when the samesequence is transmitted from different user equipments at the sametiming with the same frequency, there is a possibility of collision ofrandom access preambles, which causes a random access to fail.

Next, the target base station which has received the random accesspreamble determines a timing adjustment value for each user equipmentaccording to the reception timing (step S54). For example, the randomaccess preamble of a user equipment which is located farther from thetarget base station arrives at the target base station at the latertiming than the random access preamble of a user equipment which islocated closer to the target base station. In this case, the target basestation allocates a timing adjustment value indicating that a signalshould be transmitted earlier to the former (i.e. far) user equipment.Further, the target base station performs scheduling for a connectionrequest of the user equipment. Note that, when a communication resourcefor a connection request cannot be allocated due to the lack ofavailable communication resources, a random access fails.

Then, the target base station transmits a random access response to theuser equipment (step S56). The random access response contains thetiming adjustment value determined by the target base station andscheduling information for a connection request by the user equipment.Receiving the random access response, the user equipment adjusts thetransmission timing of a signal to the target base station by using thetiming adjustment value contained in the random access response (stepS58).

Then, the user equipment transmits a connection request to the targetbase station by using the communication resource which is specified bythe scheduling information contained in the random access response (stepS60). The connection request contains identification information of theuser equipment or the like. Note that, for example, there is apossibility that the same random access preamble is transmitted from twoor more user equipments in the step S52, and the random access preambleis received normally by the target base station. In such a case, the twoor more user equipments which have received a random access responsetransmitted from the target base station can simultaneously transmitconnection requests in the step S60. In this case also, a collisionoccurs between the connection requests, and a random access can therebyfail.

The target base station which has received the connection request fromthe user equipment then transmits acknowledgement to the user equipment(step S62). Note that, when a communication resource for theacknowledgement cannot be allocated due to the lack of availablecommunication resources, there is a possibility that the acknowledgementis not transmitted. When the user equipment receives the acknowledgementin the step S62, it recognizes that the random access has succeeded, anda series of random access procedure ends. On the other hand, when theuser equipment does not receive the random access response or theacknowledgement, it determines that the random access has failed andretries the random access after the lapse of a random backoff period.

FIG. 3B shows a procedure of a contention-free random access. Referringto FIG. 3B, the target base station first allocates a random accesspreamble to the user equipment (step S72). The random access preamblefor the contention-free random access is selected by the target basestation from a predetermined number (e.g. reserved 10) of sequencesamong 64 orthogonal sequences, for example.

Next, the user equipment transmits the random access preamble to thetarget base station (step S74). In this case, because there is no casewhere the same random access preamble is used by different userequipments at the same timing with the same frequency, a collision ofrandom access preambles does not occur.

Then, the target base station which has received the random accesspreamble determines a timing adjustment value for each user equipmentaccording to the reception timing (step S76). Further, the target basestation performs scheduling for a connection request of the userequipment. Note that, when a communication resource for a connectionrequest cannot be allocated due to the lack of available communicationresources, a random access fails.

Then, the target base station transmits a random access response to theuser equipment (step S78). The random access response contains thetiming adjustment value determined by the target base station andscheduling information for a connection request by the user equipment.Receiving the random access response, the user equipment adjusts thetransmission timing of a signal to the target base station by using thetiming adjustment value contained in the random access response (stepS80). After that, the user equipment establishes a connection with thetarget base station in the same manner as the contention-based randomaccess procedure.

As is understood from the above description, in the random accessprocedure, a random access can fail due to the collision of signals orthe shortage of communication resources. Then, a random access isretried after the lapse of a random backoff period. During this period,a communication by the user equipment delays. Further, the time neededfor calculation of a timing adjustment value and adjustment oftransmission timing is the overhead of a communication. The use of thecontention-free random access procedure reduces the possibility ofsignal collision. However, because the number of random access preambleswhich are reserved for the contention-free random access is limited, thecontention-free random access procedure is not always available in ahandover procedure. Therefore, for the purpose of preventing thedegradation of service quality, it is effective to perform a randomaccess during a handover procedure in a radio communication involvingthe carrier aggregation more efficiency as in two embodiments of thepresent invention which are described in detail below.

<2. Outline of Radio Communication System>

FIG. 4 is a schematic view showing an outline of a radio communicationsystem 1 according to an embodiment of the present invention. Referringto FIG. 4, the radio communication system 1 includes a user equipment100, a base station 200 a and a base station 200 b. It is assumed thatthe base station 200 a is a serving base station for the user equipment100.

The user equipment 100 is located inside a cell 202 a where a radiocommunication service is provided by the base station 200 a. The userequipment 100 can perform a data communication with another userequipment (not shown) via the base station 200 a over a communicationchannel formed by aggregating a plurality of component carriers (i.e. bycarrier aggregation). However, because the distance between the userequipment 100 and the base station 200 a is not short, there is apossibility that a handover is required for the user equipment 100.Further, the user equipment 100 is located inside a cell 202 b where aradio communication service is provided by the base station 200 b.Therefore, the base station 200 b can be a candidate for a target basestation for a handover of the user equipment 100.

The base station 200 a can communicate with the base station 200 bthrough a backhaul link (e.g. X2 interface). Various kinds of messagesin the handover procedure as described with reference to FIG. 1,scheduling information related to the user equipment belonging to eachcell or the like, for example, can be transmitted and received betweenthe base station 200 a and the base station 200 b. Further, the basestation 200 a and the base station 200 b can communicate with the MME,which is an upper node, through S1 interface, for example.

It should be noted that, when there is no particular need to distinguishbetween the base station 200 a and the base station 200 b in thefollowing description of the specification, they are collectivelyreferred to as a base station 200 by omitting the alphabetical letter atthe end of the reference symbol. The same applies to the other elements.

<3. Description of First Embodiment>

A first embodiment of the present invention for making a more efficientrandom access during a handover procedure in a radio communicationinvolving the carrier aggregation is described hereinafter withreference to FIGS. 5 to 9B.

(3-1. Exemplary Configuration of User Equipment)

FIG. 5 is a block diagram showing an example of a configuration of theuser equipment 100 according to the embodiment. Referring to FIG. 5, theuser equipment 100 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 160, an adjustment value storageunit 162, and a measurement unit 170.

(Radio Communication Unit)

The radio communication unit 110 performs a radio communication with thebase station 200 over a communication channel formed by aggregating aplurality of component carriers with use of the carrier aggregationtechnology.

FIG. 6 is a block diagram showing an example of a more detailedconfiguration of the radio communication unit 110. Referring to FIG. 6,the radio communication unit 110 includes an antenna 112, a switch 114,a low noise amplifier (LNA) 120, a plurality of down-converters 122 a to122 c, a plurality of filters 124 a to 124 c, a plurality ofanalogue-to-digital converters (ADCs) 126 a to 126 c, a demodulationunit 128, a modulation unit 130, a plurality of digital-to-analogueconverters (DACs) 132 a to 132 c, a plurality of filters 134 a to 134 c,a plurality of up-converters 136 a to 136 c, a combiner 138, and a poweramplifier (PA) 140.

The antenna 112 receives a radio signal transmitted from the basestation 200 and outputs the received signal to the LNA 120 through theswitch 114. The LNA 120 amplifies the received signal. Thedown-converter 122 a and the filter 124 a separate a baseband signal ofthe first component carrier (CC1) from the received signal amplified bythe LNA 120. Then, the separated baseband signal is converted to adigital signal by the ADC 126 a and output to the demodulation unit 128.Likewise, the down-converter 122 b and the filter 124 b separate abaseband signal of the second component carrier (CC2) from the receivedsignal amplified by the LNA 120. Then, the separated baseband signal isconverted to a digital signal by the ADC 126 b and output to thedemodulation unit 128. Further, the down-converter 122 c and the filter124 c separate a baseband signal of the third component carrier (CC3)from the received signal amplified by the LNA 120. Then, the separatedbaseband signal is converted to a digital signal by the ADC 126 c andoutput to the demodulation unit 128. After that, the demodulation unit128 generates a data signal by demodulating the baseband signals of therespective component carriers and outputs the data signal to the signalprocessing unit 150.

Further, when a data signal is input from the signal processing unit150, the modulation unit 130 modulates the data signal and generatesbaseband signals of the respective component carriers. Among thosebaseband signals, the baseband signal of the first component carrier(CC1) is converted to an analog signal by the DAC 132 a. Then, afrequency component corresponding to the first component carrier in atransmission signal is generated from the analog signal by the filter134 a and the up-converter 136 a. Likewise, the baseband signal of thesecond component carrier (CC2) is converted to an analog signal by theDAC 132 b. Then, a frequency component corresponding to the secondcomponent carrier in the transmission signal is generated from theanalog signal by the filter 134 b and the up-converter 136 b. Further,the baseband signal of the third component carrier (CC3) is converted toan analog signal by the DAC 132 c. Then, a frequency componentcorresponding to the third component carrier in the transmission signalis generated from the analog signal by the filter 134 c and theup-converter 136 c. After that, the generated frequency componentscorresponding to the three component carriers are combined by thecombiner 138, and the transmission signal is formed. The PA 140amplifiers the transmission signal and outputs the transmission signalto the antenna 112 through the switch 114. Then, the antenna 112transmits the transmission signal as a radio signal to the base station200.

Although the case where the radio communication unit 110 handles threecomponent carriers is described in FIG. 6, the number of componentcarriers handled by the radio communication unit 110 may be two, or fouror more.

Further, instead of processing the signals of the respective componentcarriers in the analog region as in the example of FIG. 6, the radiocommunication unit 110 may process the signals of the respectivecomponent carriers in the digital region. In the latter case, at thetime of reception, a digital signal converted by one ADC is separatedinto the signals of the respective component carriers by a digitalfilter. Further, at the time of transmission, after digital signals ofthe respective component carriers are frequency-converted and combined,the signal is converted into an analog signal by one DAC. The load ofthe ADC and the DAC is generally smaller when processing the signals ofthe respective component carriers in the analog region. On the otherhand, when processing the signals of the respective component carriersin the digital region, a sampling frequency for AD/DA conversion ishigher, and the load of the ADC and the DAC can thereby increase.

(Signal Processing Unit)

Referring back to FIG. 5, an example of a configuration of the userequipment 100 is further described below.

The signal processing unit 150 performs signal processing such asdeinterleaving, decoding or error correction on the demodulated datasignal that is input from the radio communication unit 110. Then, thesignal processing unit 150 outputs the processed data signal to an upperlayer. Further, the signal processing unit 150 performs signalprocessing such as encoding or interleaving on the data signal that isinput from the upper layer. Then, the signal processing unit 150 outputsthe processed data signal to the radio communication unit 110.

(Control Unit)

The control unit 160 controls the overall functions of the userequipment 100 by using a processing device such as a central processingunit (CPU) or a digital signal processor (DSP). For example, the controlunit 160 controls the timing of data communication by the radiocommunication unit 110 according to scheduling information that isreceived from the base station 200 by the radio communication unit 110.At this time, the control unit 160 adjusts the timing of datatransmission from the radio communication unit 110 by using a timingadjustment value which is notified from the base station 200 in a randomaccess procedure. Further, the control unit 160 controls the measurementunit 170 to measure the channel quality by using a reference signal fromthe base station 200, which is a serving base station, and transmits thechannel quality report to the base station 200 through the radiocommunication unit 110. Further, the control unit 160 controls themeasurement unit 170 to execute measurement during the periods ofmeasurement gaps which are allocated by the base station 200.

Further, in this embodiment, when two or more component carriers areallocated by the target base station during a handover procedure, thecontrol unit 160 determines whether a distance in the frequencydirection between the two or more component carriers is smaller than aspecific threshold. When the distance in the frequency direction issmaller than a specific threshold, the control unit 160 skips a randomaccess to the target base station for at least one of the two or morecomponent carriers. Specifically, when first and second componentcarriers exist in close proximity in the frequency direction, thecontrol unit 160 makes a random access for the first component carrierand skips a random access for the second component carrier. The specificthreshold may be the maximum value of a difference between twofrequencies (e.g. the center frequencies of the respective componentcarriers) whose transmission timing can be adjusted appropriately basedon one timing adjustment value notified from the target base station ina random access procedure. Thus, even when a random access is skippedfor any of the component carriers, the user equipment 100 can carry outa handover by appropriately adjusting the transmission timing for thecomponent carrier. A transmission timing difference depends on adistance between each user equipment and a base station. Thus, a timingadjustment is typically used between user equipment and a base stationfor purposes of synchronization. When the user equipment handles aplurality of component carriers, it is also typical for the userequipment to adjust transmission timing for each component carrierbecause the timing difference depends on frequency. Generally,propagation characteristics include, but not limited to, characteristicsof reflection, diffraction, scattering and propagation. Thesecharacteristics are known to depend on frequency of the componentcarrier. Therefore, if one frequency is close another frequency, thecharacteristic such as propagation delay is similar to each otherbetween two different frequencies. These propagation characteristicsaffect the propagation delay amount of the signals. Therefore, in thecase where the a distance in the frequency direction between the twocomponent carriers is smaller than a specific threshold, propagationdelay of the signals over the two component carriers is close to eachother. Therefore, it becomes easier to adjust transmission timing of onecomponent carrier based on the transmission timing of another componentcarrier.

Further, in the case where a new communication channel with a targetbase station is composed of three or more component carriers, when adistance in the frequency direction between certain component carriersamong them is smaller than a specific threshold, the control unit 160may skip a random access for at least one of the component carriers. Forexample, it is assumed that a distance in the frequency directionbetween the component carrier CC1 and the component carrier CC2 amongthe component carriers CC1 to CC3 is smaller than a specific threshold,and a distance between the component carrier CC1, CC2 and the componentcarrier CC3 is larger than the specific threshold. In this case, thecontrol unit 160 may make a random access to the target base station forthe component carriers CC1 and CC2 in one time and further make a randomaccess to the target base station for the component carrier CC3.

In the case of skipping a random access for any of the componentcarriers, the control unit 160 first executes a random access to thetarget base station for another component carrier which exists in closeproximity to the relevant component carrier in the frequency direction.Then, the control unit 160 stores the timing adjustment value which isnotified from the target base station in the wake of the random accessinto the adjustment value storage unit 162. After that, the control unit160 adjusts the transmission timing for the component carrier for whicha random access is skipped by using the timing adjustment value storedin the adjustment value storage unit 162. Then, the control unit 160makes an access to the target base station over the relevant componentcarrier at the adjusted transmission timing (the access in this case isnot a random access).

(Adjustment Value Storage Unit)

The adjustment value storage unit 162 stores a timing adjustment valuefor one or more component carriers which is input from the control unit160 by using a storage medium such as a hard disk or semiconductormemory. Then, in response to a command from the control unit 160, theadjustment value storage unit 162 outputs the stored timing adjustmentvalue for use in adjustment of the transmission timing for anothercomponent carrier.

(Measurement Unit)

The measurement unit 170 measures the channel quality for each of thecomponent carriers by using a reference signal from the base station 200according to control from the control unit 160, for example. Further,the measurement unit 170 executes measurement for a handover withrespect to each of the component carriers by using the measurement gapswhich are allocated by the base station 200. A result of the measurementexecuted by the measurement unit 170 is converted to a predeterminedformat for a measurement report by the control unit 160 and transmittedto the base station 200 through the radio communication unit 110. Afterthat, the base station 200 determines, based on the measurement report,whether a handover should be executed or not for the user equipment 100.

(3-2. Exemplary Configuration of Base Station)

FIG. 7 is a block diagram showing an example of a configuration of thebase station 200 according to the embodiment. Referring to FIG. 7, thebase station 200 includes a radio communication unit 210, an interfaceunit 250, a component carrier (CC) management unit 260, and a controlunit 280.

(Radio Communication Unit)

A specific configuration of the radio communication unit 210 may besimilar to the configuration of the radio communication unit 110 of theuser equipment 100 which is described above with reference to FIG. 6,although the number of component carriers to be supported, therequirements of processing performance or the like are different. Theradio communication unit 210 performs a radio communication with theuser equipment over a communication channel which is formed byaggregating a plurality of component carriers with use of the carrieraggregation technology.

(Interface Unit)

The interface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and an upper node throughthe S1 interface illustrated in FIG. 4, for example. Further, theinterface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and another base stationthrough the X2 interface illustrated in FIG. 4, for example.

(CC Management Unit)

The CC management unit 260 holds data that indicates which componentcarrier each user equipment is using for communication with respect toeach of the user equipments belonging to the cell of the base station200. Such data can be updated by the control unit 280 when an additionaluser equipment joins the cell of the base station 200 or when theexisting user equipment changes its component carriers. Thus, thecontrol unit 280 can recognize which component carrier the userequipment 100 is using by referring to the data held by the CCmanagement unit 260.

(Control Unit)

The control unit 280 controls the overall functions of the base station200 by using a processing device such as a CPU or a DSP. For example,the control unit 280 executes an allocation of communication resourcesfor data communication, i.e. scheduling, for the user equipment 100 andother user equipments. Then, the control unit 280 delivers schedulinginformation over a broadcast channel in a given sub-frame. Further, whenthe base station 200 is a target base station of a handover of a radiocommunication involving the carrier aggregation, the control unit 280allocates two or more component carriers to a new communication channel.At this time, when two or more component carriers which are adjacent inthe frequency direction are allocable, the control unit 280 allocatesthe two or more component carriers to the new communication channel.Further, when two or more component carriers whose distance in thefrequency direction is smaller than the above-described specificthreshold are allocable, the control unit 280 allocates the two or morecomponent carriers to the new communication channel.

FIG. 8 is an explanatory view to describe an allocation of componentcarriers by a target base station according to the embodiment. FIG. 8shows a step-by-step handover including four stages in the case where acommunication channel is composed of three component carriers CC1 toCC3. The first stage is before a handover. The second stage is after ahandover of the component carrier CC1. The third stage is after ahandover of the component carriers CC1 and CC2. The fourth stage isafter a handover of all the component carriers.

First, in the first stage, the component carriers CC1 to CC3 areconnected with a source base station. The positions of the componentcarriers CC1 to CC3 in the frequency direction may be any positions.

Next, when a handover is completed for the component carrier CC1, thehandover procedure proceeds to the second stage. The frequency band towhich the component carrier CC1 is allocated in the target base stationmay be the same frequency band as the frequency band in the source basestation or a different frequency band from the frequency band in thesource base station.

Then, the control unit 280 of the target base station which has receiveda handover request for the component carrier CC2 determines whether thecomponent carriers CC1 and CC2 can be allocated so that they areadjacent in the frequency direction. It is assumed in the example ofFIG. 8 that the component carriers CC1 and CC2 can be allocated so thatthey are adjacent in the frequency direction. Thus, the control unit 28allocates the component carriers CC1 and CC2 so that they are adjacentin the frequency direction (third stage).

Further, the control unit 280 of the target base station which hasreceived a handover request for the component carrier CC3 determineswhether the component carrier CC1 or CC2 and the component carrier CC3can be allocated so that they are adjacent in the frequency direction.It is assumed in the example of FIG. 8 that those component carriers arenot allocable to be adjacent in the frequency direction. Then, thecontrol unit 28 determines whether the component carrier CC3 can beallocated to a new communication channel so that a distance from thecomponent carrier CC1 or CC2 in the frequency direction is smaller thana threshold F_(TH). The threshold F_(TH) indicates the above-describedspecified threshold which can correspond to the maximum value of adifference between two frequencies whose transmission timing can beadjusted appropriately based on one timing adjustment value. It isassumed in the example of FIG. 8 that the component carrier CC3 can beallocated so that the distances from the component carriers CC1 and CC2are smaller than the threshold F_(TH). Thus, the control unit 280allocates the component carrier CC3 to the position where the distancesfrom the component carriers CC1 and CC2 are smaller than the thresholdF_(TH) (fourth stage).

It should be noted that, FIG. 8 shows the case where, after a handoverfor one component carrier is completed, another component carrier isallocated to the new communication channel. However, the allocation ofanother component carrier to a new communication channel may beperformed before completion of a handover for one component carrier.Further, in the example of FIG. 8, it is described that, for a certaincomponent carrier, the frequency band of the component carrier ischanged at the time of an access to the target base station by the userequipment 100. However, for a certain component carrier, a base stationto which the component carrier is connected may be changed from thesource base station to the target base station after the frequency bandhas changed in the source base station according to the above-describedconditions related to the position in the frequency direction.

In addition to such an allocation of component carriers, the controlunit 280 controls the base station 200 to operate in the same manner asthe source base station or the target base station in the handoverprocedure which is descried with reference to FIG. 1.

(3-3. Flow of Process)

A flow of a handover procedure according to the embodiment is describedhereinafter with reference to FIGS. 9A and 9B. Note that, in thefollowing scenario, it is assumed that a handover procedure is performedamong the user equipment 100, the base station 200 a serving as a sourcebase station, and the base station 200 b serving as a target basestation. Further, for simplification of description, it is assumed inthis scenario that the user equipment 100 performs a radio communicationby using two component carriers. Furthermore, for the procedure up tomeasurement in the user equipment (steps S2 to S14) in the typicalhandover procedure illustrated in FIG. 1, explanation is omitted becausethere is no significant difference.

Referring to FIG. 9A, the user equipment 100 first transmits ameasurement report for the component carrier CC1, for example, to thebase station 200 a (step S122). Receiving the measurement report, thebase station 200 a determines the necessity of a handover based on themeasurement report. For example, when a channel quality between the userequipment 100 and the base station 200 b is better than a channelquality between the user equipment 100 and the base station 200 a by apredetermined threshold or greater, it can be determined that a handoveris necessary. In this case, the base station 200 a transmits a handoverrequest message for the component carrier CC1 to the base station 200 b(step S124). Receiving the handover request message, the base station200 b allocates the component carrier CC1 to any frequency band for anew communication channel with the user equipment 100 according to theavailability of a communication service. Then, the base station 200 btransmits a handover request confirm message to the base station 200 a(step S126). Receiving the handover request confirm message, the basestation 200 a transmits a handover command for the component carrier CC1to the user equipment 100 (step S128).

Receiving the handover command, the user equipment 100 first acquiressynchronization with the downlink channel of the component carrier CC1of the base station 200 b (step S132). Then, the user equipment 100makes a random access to the base station 200 b by using a random accesschannel of the component carrier CC1 (step S134). During this period,the control unit 160 of the user equipment 100 stores the timingadjustment value for the component carrier CC1 which is notified fromthe base station 200 b into the adjustment value storage unit 162.During the downlink synchronization and the random access, the basestation 200 a forwards data addressed to the user equipment 100 to thebase station 200 b (step S136).

Then, after success in the random access for the component carrier CC1,the user equipment 100 transmits a handover complete message for thecomponent carrier CC1 to the base station 200 b (step S142). Receivingthe handover complete message, the base station 200 b requests the MMEto perform route update for the component carrier CC1 of the userequipment 100 (step S144). Upon updating the route of user data by theMME, the user equipment 100 becomes able to communicate with anotherdevice through a new base station (i.e. the base station 200 b). Notethat the request for route update may be performed with respect to eachcomponent carrier, or performed only once through a plurality ofcomponent carriers. Then, the base station 200 b transmitsacknowledgement for the handover complete message to the user equipment100 (step S146).

Further, referring to FIG. 9B, the user equipment 100 transmits ameasurement report for the component carrier CC2 to the base station 200a (step S152). Receiving the measurement report, the base station 200 atransmits a handover request message for the component carrier CC2 tothe base station 200 b (step S154).

Receiving the handover request message, the base station 200 b allocatesthe component carrier CC2 to any frequency band for the newcommunication channel with the user equipment 100 according to theavailability of a communication service. In this scenario, it is assumedthat the frequency band to which the component carrier CC2 is allocatedis the frequency band which is adjacent to the component carrier CC1 orthe frequency band in which the distance from the component carrier CC1in the frequency direction is smaller than the threshold F_(TH). Afterthat, the base station 200 b transmits a handover request confirmmessage to the base station 200 a (step S156). Receiving the handoverrequest confirm message, the base station 200 a transmits a handovercommand for the component carrier CC2 to the user equipment 100 (stepS158).

Then, in the user equipment 100 which has received the handover command,the control unit 160 reads the timing adjustment value for the componentcarrier CC1 which is stored in the adjustment value storage unit 162.The control unit 160 then adjusts the access timing to the base station200 b for the component carrier CC2 by using the timing adjustment valuewhich is read from the adjustment value storage unit 162 (step S164).During this period, the base station 200 a forwards data addressed tothe user equipment 100 to the base station 200 b (step S166).

The user equipment 100 then transmits a handover complete message forthe component carrier CC2 to the base station 200 b (step S172).Receiving the handover complete message, the base station 200 b requeststhe MME to perform route update for the component carrier CC2 of theuser equipment 100 (step S174).

Then, the base station 200 b transmits acknowledgement for the handovercomplete message to the user equipment 100 (step S176).

(3-4. Summary of First Embodiment)

The first embodiment of the present invention is described above withreference to FIGS. 5 to 9B. According to the embodiment, in a radiocommunication involving the carrier aggregation, when a distance in thefrequency direction between two or more component carriers which areallocated to a communication channel with a target base station issmaller than a specific threshold, the user equipment 100 skips a randomaccess to the target base station for at least one of the two or morecomponent carriers. Therefore, an accumulation of delays correspondingto the number of component carriers due to a failure of a random accessdoes not occur, and the degradation of service quality caused by arandom access during a handover procedure is suppressed. Further, theuser equipment 100 adjusts the transmission timing for the componentcarrier for which a random access has been skipped by using the timingadjustment value notified from the target base station for the componentcarrier for which a random access has been made. A communication can bethereby performed at appropriate timing also in the component carrierfor which a random access has been skipped. Further, because it is notnecessary in the target base station to redundantly determine the timingadjustment value for the component carrier, the overhead of acommunication is reduced.

Further, according to the embodiment, when two or more componentcarriers which are adjacent in the frequency direction are allocable,the base station 200 serving as a target base station allocates the twoor more component carriers to a new communication channel with the userequipment 100. Further, when two or more component carriers whosedistance in the frequency direction is smaller than a specific thresholdare allocable, the base station 200 allocates the two or more componentcarriers to a new communication channel. The opportunity of skipping arandom access by the user equipment 100 during a handover procedurethereby increases, and it is possible to more effectively obtain theabove-described advantages such as prevention of an accumulation ofdelays due to a failure of a random access or reduction of overhead.Note that the positions in the frequency direction of those componentcarriers may be varied after the end of a series of handover procedureas a matter of course.

<4. Description of Second Embodiment>

A second embodiment of the present invention for making a more efficientrandom access during a handover procedure in a radio communicationinvolving the carrier aggregation is described hereinafter withreference to FIGS. 10 to 12B.

(4-1. Exemplary Configuration of User Equipment)

FIG. 10 is a block diagram showing an example of a configuration of theuser equipment 300 according to the embodiment. Referring to FIG. 10,the user equipment 300 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 360, and a measurement unit 170.

(Control Unit)

The control unit 360 controls the overall functions of the userequipment 300 by using a processing device such as a CPU or a DSP. Forexample, the control unit 360 controls the timing of data communicationby the radio communication unit 110 according to scheduling informationthat is received from the base station 400 by the radio communicationunit 110. Further, the control unit 360 controls the measurement unit170 to measure the channel quality by using a reference signal from thebase station 400, which is a serving base station, and transmits thechannel quality report to the base station 400 through the radiocommunication unit 110. Further, the control unit 360 controls themeasurement unit 170 to execute measurement during the periods ofmeasurement gaps which are allocated by the base station 400.

Further, in this embodiment, when one or more component carriers areallocated to a new communication channel by the base station 400 duringa handover procedure, the control unit 360 controls the radiocommunication unit 110 to make a random access to the base station 400.By a random access for the one or more component carriers, a newcommunication channel with the base station 400 is established. Then,the control unit 360 controls the radio communication unit 110 to notifythe base station 400 of the timing of a random access for one or moreother (remaining) component carriers to constitute the communicationchannel over the established new communication channel. The base station400 as a target base station can thereby reserve communication resourcesnecessary in the random access procedure by the user equipment 300 inadvance based on the notified timing.

(4-2. Exemplary Configuration of Base Station]

FIG. 11 is a block diagram showing an example of a configuration of thebase station 400 according to the embodiment. Referring to FIG. 11, thebase station 400 includes a radio communication unit 210, an interfaceunit 250, a CC management unit 260, and a control unit 480.

(Control Unit)

The control unit 480 controls the overall functions of the base station400 by using a processing device such as a CPU or a DSP. For example,the control unit 480 executes an allocation of communication resourcesfor data communication for the user equipment 300 and other userequipments. Then, the control unit 480 delivers scheduling informationover a broadcast channel in a given sub-frame. Further, when the basestation 400 is a target base station of a handover by the user equipment300, the control unit 480 sequentially allocates two or more componentcarriers to a new communication channel with the user equipment 300. Atthis time, over the new communication channel which is established by arandom access for one or more component carriers, the timing of a randomaccess for one or more other component carriers to constitute thecommunication channel can be notified from the user equipment 300 asdescribed above. Receiving such a notification, the control unit 480reserves communication resources for the user equipment 300 according tothe notified timing so that the subsequent random access for the one ormore other component carriers succeeds. The communication resourcesreserved thereby include resources of an uplink for a connection requestfrom the user equipment 300, resources of a downlink for acknowledgementto the connection request or the like.

In addition to such a reservation of communication resources, thecontrol unit 480 controls the base station 400 to operate in the samemanner as the source base station or the target base station in thehandover procedure which is descried with reference to FIG. 1.

(4-3. Flow of Process)

A flow of a handover procedure according to the embodiment is describedhereinafter with reference to FIGS. 12A and 12B. Note that, in thefollowing scenario, it is assumed that a handover procedure is performedamong the user equipment 300, the base station 400 a serving as a sourcebase station, and the base station 400 b serving as a target basestation. Further, like FIGS. 9A and 9B, for simplification ofdescription, it is assumed in this scenario that the user equipment 300performs a radio communication by using two component carriers.Furthermore, for the procedure up to measurement in the user equipment(steps S2 to S14) in the typical handover procedure illustrated in FIG.1, explanation is omitted because there is no significant difference.

Referring to FIG. 12A, the user equipment 300 first transmits ameasurement report for the component carrier CC1, for example, to thebase station 400 a (step S222). Receiving the measurement report, whenthe base station 400 a determines that a handover is necessary, the basestation 400 a transmits a handover request message for the componentcarrier CC1 to the base station 400 b (step S224). Receiving thehandover request message, the base station 400 b allocates the componentcarrier CC1 to any frequency band for a new communication channel withthe user equipment 300 according to the availability of a communicationservice. Then, the base station 400 b transmits a handover requestconfirm message to the base station 400 a (step S226). Receiving thehandover request confirm message, the base station 400 a transmits ahandover command for the component carrier CC1 to the user equipment 300(step S228).

Receiving the handover command, the user equipment 300 first acquiressynchronization with the downlink channel of the component carrier CC1of the base station 400 b (step S232). Then, the user equipment 300makes a random access to the base station 400 b by using a random accesschannel of the component carrier CC1 (step S234). When the random accessis success, a new communication channel between the user equipment 300and the base station 400 b is established. During the downlinksynchronization and the random access, the base station 400 a forwardsdata addressed to the user equipment 300 to the base station 400 b (stepS236).

Then, after success in the random access for the component carrier CC1,the user equipment 300 transmits a handover complete message for thecomponent carrier CC1 to the base station 400 b (step S242). Receivingthe handover complete message, the base station 400 b requests the MMEto perform route update for the component carrier CC1 of the userequipment 300 (step S244). Then, the base station 400 b transmitsacknowledgement for the handover complete message to the user equipment300 (step S246).

Further, referring to FIG. 12B, the user equipment 300 transmits ameasurement report for the component carrier CC2 to the base station 400a (step S252). Receiving the measurement report, the base station 400 atransmits a handover request message for the component carrier CC2 tothe base station 400 b (step S254).

Receiving the handover request message, the base station 400 b allocatesthe component carrier CC2 to any frequency band for the newcommunication channel with the user equipment 300 according to theavailability of a communication service. The frequency band to which thecomponent carrier CC2 is allocated may be any frequency band. Afterthat, the base station 400 b transmits a handover request confirmmessage to the base station 400 a (step S256). Receiving the handoverrequest confirm message, the base station 400 a transmits a handovercommand for the component carrier CC2 to the user equipment 300 (stepS258).

Then, the user equipment 300 notifies the timing of a random access (RA)for the component carrier CC2 to the base station 400 b over theestablished new communication channel (which has the component carrierCC1) (step S260). The base station 400 b then reserves communicationresources according to the notified timing so that a random access forthe component carrier CC2 by the user equipment 300 does not fail due tothe shortage of communication resources (step S261).

After that, the user equipment 300 acquires synchronization with thedownlink channel of the component carrier CC2 of the base station 400 b(step S262). Then, the user equipment 300 makes a random access to thebase station 400 b by using a random access channel of the componentcarrier CC2 at the timing notified to the base station 400 b in the stepS260 (step S264). Such a random access is likely to succeed unlesssignal collision occurs because communication resources for a connectionrequest from the user equipment 300 and acknowledgement for the requestare reserved by the base station 400 b. During the downlinksynchronization and the random access, the base station 400 a forwardsdata addressed to the user equipment 300 to the base station 400 b (stepS266).

The user equipment 300 then transmits a handover complete message forthe component carrier CC2 to the base station 400 b (step S272).Receiving the handover complete message, the base station 400 b requeststhe MME to perform route update for the component carrier CC2 of theuser equipment 300 (step S274). Then, the base station 400 b transmitsacknowledgement for the handover complete message to the user equipment300 (step S276).

(4-4. Summary of Second Embodiment)

The second embodiment of the present invention is described above withreference to FIGS. 10 to 12B. According to the embodiment, at the timeof a handover in a radio communication involving the carrieraggregation, the timing of a random access for a component carrier forwhich a handover has not been completed is notified from the userequipment 300 to the base station 400 over a component carrier for whicha handover has been completed. The base station 400 can thereby reservecommunication resources so that the subsequent random access for anothercomponent carrier from the user equipment 300 does not fail due to theshortage of communication resources. As a result, the possibility of afailure of a random access is reduced, and the degradation of servicequality due to an accumulation of delays is suppressed.

Although preferred embodiments of the present invention are described indetail above with reference to the appended drawings, the presentinvention is not limited thereto. It should be understood by thoseskilled in the art that various modifications, combinations,sub-combinations and alterations may occur depending on designrequirements and other factors insofar as they are within the scope ofthe appended claims or the equivalents thereof.

REFERENCE SIGNS LIST

-   1 Radio Communication System-   100, 300 USER EQUIPMENT-   110 Radio Communication Unit (User Equipment)-   160, 360 Control Unit (User Equipment)-   200, 400 Base Station-   210 Radio Communication Unit (Base Station)-   280, 480 Control Unit (Base Station)

1. (canceled)
 2. A mobile station in a wireless communication network,comprising: a radio communication interface configured to communicatewith a base station via a communication channel formed by aggregating aplurality of component carriers; processing circuitry configured tocontrol a handover from a first base station to a second base station bythe radio communication interface, and to skip a random access, to thesecond base station, on at least one of two or more component carriers;and an adjustment value memory configured to store a timing adjustmentvalue notified for a component carrier of the two or more componentcarriers on which random access is carried out; wherein the processingcircuitry is configured to adjust transmission timing on a componentcarrier on which a random access has been skipped by using the timingadjustment value stored in the memory.
 3. The mobile station of claim 2,wherein the processing circuitry is configured to acquiresynchronization with the base station via the component carrier forwhich random access is carried out, and the radio communicationinterface is configured to transmit an access request message based onthe synchronization.
 4. The mobile station of claim 2, wherein thetiming adjustment value is based on a signal propagation characteristicbetween the mobile station and the base station.
 5. The mobile stationof claim 2, wherein the timing adjustment value depends on a frequencyof the component carrier for which random access is carried out.
 6. Themobile station of claim 2, wherein the two or more component carriersare in closest proximity to each other relative to other availablecomponent carriers.
 7. The mobile station of claim 2, wherein the two ormore component carriers are adjacent to each other.
 8. The mobilestation of claim 2, wherein the two or more component carriers arewithin a predetermined threshold of each other.
 9. A base station in awireless communication network, the base station comprising: a radiocommunication interface configured to communicate with a mobile stationvia a communication channel formed by aggregating a plurality ofcomponent carriers; processing circuitry configured to control ahandover of the mobile station from another base station, wherein, theprocessing circuitry is configured to allocate two or more componentcarrier, and notify timing adjustment value to the mobile station for acomponent carrier of the two or more component carriers on which themobile station carries out random access, and wherein the radiocommunication interface is configured to receive information, about acomponent carrier of the two or more component carriers for which randomaccess is skipped, transmitted from the mobile station on a transmissiontiming adjusted by the mobile station using the timing adjustment valuenotified for the component carrier on which the mobile station carriesout random access.
 10. The base station of claim 9, wherein theprocessing circuitry is configured to assign the two or more componentcarriers to be adjacent to each other.
 11. The base station of claim 9,wherein the processing circuitry is configured to assign the two or morecomponent carriers be within a predetermined threshold to each other.12. The base station of claim 9, wherein the processing circuitry isconfigured to assign a second component carrier of the two or morecomponent carriers, which is one of a plurality of available componentcarriers closest in proximity to a first component carrier of the two ormore component carriers.
 13. A wireless communication system comprising:a mobile station; a first base station; and a second base station,wherein the mobile station includes: a first radio communicationinterface configured to communicate with the second base station via acommunication channel formed by aggregating a plurality of componentcarriers; and a first control unit configured to control a handover fromthe first base station to the second base station by the first radiocommunication interface, and to skip a random access, to the second basestation, on at least one of two or more component carriers; and anadjustment value memory configured to store a timing adjustment valuenotified for a component carrier of the two or more component carrierson which random access is carried out; wherein the first control unit isconfigured to adjust transmission timing on a component carrier on whicha random access has been skipped by using the timing adjustment valuestored in the adjustment value memory, and the second base stationincludes: a second radio communication interface configured tocommunicate with a mobile station via a communication channel formed byaggregating a plurality of component carriers; a second control unitconfigured to control a handover of the mobile station from the firstbase station, wherein, the second control unit is configured to allocatetwo or more component carrier, and notify timing adjustment value to themobile station for a component carrier of the two or more componentcarriers on which the mobile station carries out random access, andwherein the second radio communication interface is configured toreceive information, about a component carrier of the two or morecomponent carriers for which random access is skipped, transmitted fromthe mobile station on a transmission timing adjusted by the mobilestation using the timing adjustment value notified for the componentcarrier on which the mobile station carries out random access.
 14. Amethod executed by a mobile station in a wireless communication networkcomprising: communicating with a base station via a communicationchannel formed by aggregating a plurality of component carriers;controlling a handover from a first base station to a second basestation; skipping a random access, to the second base station, on atleast one of two or more component carriers; storing a timing adjustmentvalue notified for a component carrier of the two or more componentcarriers on which random access is carried out; and adjustingtransmission timing on a component carrier on which a random access hasbeen skipped by using the stored timing adjustment value.
 15. A methodexecuted by a base station in a wireless communication networkcomprising: communicating with a mobile station via a communicationchannel formed by aggregating a plurality of component carriers;controlling a handover of the mobile station from another base station;allocating two or more component carrier, and notify timing adjustmentvalue to the mobile station for a component carrier of the two or morecomponent carriers on which the mobile station carries out randomaccess; and receiving information, about a component carrier of the twoor more component carriers for which random access is skipped,transmitted from the mobile station on a transmission timing adjusted bythe mobile station using the timing adjustment value notified for thecomponent carrier on which the mobile station carries out random access.